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As you know, it is quite obvious that Bicycle spokes attach the hub in the centre to the rim. What else do they do? If you compare the wheels today with the ones from ancient times, there are more spokes now on motor bikes and bicycles than of a wheel of a chariot. Why is that? What effect does it have on the vehicle if there is a higher number of spokes on the wheels?

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Could those voting to close comment on their reasons? This seems to be a perfectly valid post to me. –  Emilio Pisanty Jan 17 at 14:35
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@EmilioPisanty I didn't VtC but my guess would be it's under the "Engineering" close reason. –  tpg2114 Jan 17 at 15:23
    
more spokes = more air resistance but more spokes = lighter spokes. Also better to keep the mass away from the rim, is it has a bigger effect there ×2 I think. –  richard Jan 17 at 17:36
    
Just to let anybody interested know, I've added this question to my list of interesting examples of cross-site questions on meta. –  hippietrail Jan 18 at 6:51
    
Hmm.. highly puzzling that practically and theoretically wrong answers are being upvoted! Most answers below aren't even answering the original question, let alone correctly!! Are people actually saying these answers are correct and acceptable?? This is a "physics" forum right, not a "I feel like it" forum?? Now, take a wheel, remove all the spokes, except about 3-4 near each other. Turn the wheel to point all these spokes going upward from the hub. The hub will bear weight. Try it. Turn the wheel 180 and hub won't bear weight. Tension=yes, compression=no. Proof done. See my full answer below. –  LMSingh Feb 5 at 9:39

4 Answers 4

Note that the actual mechanics of the spoke structure is somewhat non-intuitive. I hope to clarify it here.

Spokes do NOT support the weight by "bearing" the weight as it goes down from hub to bottom of the wheel. They support the weight by keeping the hub "pulled up" towards the top of the wheel. That's why they can be flexible, because they are essentially working as ropes.

For clarity.. the old style wheels with wooden peg like columns were the kind that were bearing the load (i.e. getting compressed). The bicycle and similar configurations the spokes are getting "stretched" when doing their job.

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Interestingly, none of the previous answers say that spokes support the weight by bearing the weight down from the hub to the bottom. (One strongly implies that, but the poster clarified in a comment) –  Mooing Duck Jan 17 at 20:54
    
I'm pretty sure my answer talks about compression versus tension as why spokes work and how it's different from the old style wheels. –  tpg2114 Jan 17 at 21:08
    
@tpg2114 Your answer is fundamentally different from mine. You state "..Far too thin to actually support any compressive load without buckling. .. the spokes are in tension ... A rod under tension does not buckle, so the instability is gone from the spoke". Your reply, to me seemed to be clearly making a point that the spoke is far too thin to hold weight (compression) and the only way it does so is because its "instability" is removed by tension. So as I see it you are trying to show "how a thin spoke can hold the weight and not buckle". Nice theory but buckling is irrelevant for a rope. –  LMSingh Jan 21 at 8:44
    
I missed reading @HenkLangeveld's comment before posting my answer. Noting that he is the only one who has gotten close to stating prior to my answer that the hub hangs from the top of the rim but he states "partly hangs". I want to make sure it's clear that in my view, the bike is fully, "not partly", suspended from the top portion of the rim. In other words, the only "load handling" is all done in tension and not in compression. –  LMSingh Jan 21 at 9:08

If there is weight on the axle the rim gets pushed down into the ground and tries to deform by flattening on the bottom and bulging right besides the ground. Properly tensioned spokes will counteract this bulging and lessen the deformation allowing for an easier and smoother ride.

This means that the rim does not have to be super resistant to deformation which allows it to be much lighter compared to a column-type rim.

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I would actually think that the rim gets pushed and pulled down by the weight on the axle. –  Henk Langeveld Jan 17 at 11:51
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@HenkLangeveld semantics aren't important just that the hub mostly hangs of the top half of the rim which in turn pushes the entire rim down –  ratchet freak Jan 17 at 11:53

All of the answers given are very good, and delivered well. Therefore, I have little to add to the question, but would like to mention that metallurgy has a great effect on the number of spokes needed for a given application in our modern era. The greater the tensile strength of the spoke material used; the fewer spokes that are required to transmit a given load value. Ferrari used spoked wheels on many of their early race vehicles, for the purpose of weight reduction. To insure maximum strength, while minimizing wheel weight, their engineers chose to use very thin spokes made from titanium alloy. Today, if you happen to sock a curb while making a high-speed grocery run with your vintage Ferrari, be prepared to pay in excess of $10,000 for a replacement wheel, if you can find one. Otherwise, the cost will be higher, to have one custom fabricated. Ah, the challenges presented to the unimaginably wealthy.

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In comparing wheels of today to those in history, there are traditionally more spokes now. However, that's because wheels in the past (even large wagon wheels in not-so-ancient times) used relatively thick wooden spokes that behaved like a column and dealt with the load of the wheel with compression.

However, modern spokes are very thin. Far too thin to actually support any compressive load without buckling. Modern metal spokes are very easy to bend. However, when wheels are built, the spokes are threaded into nipples and the nipples are tightened so that the spokes are in tension at all times. A rod under tension does not buckle, so the instability is gone from the spoke.

How much tension is very important of course but this isn't an answer about wheel building (although if you want information on that, hit me up in chat or ask over on Bicycles.SE, I've built many, many bicycle wheels).

The real benefit to using thin spokes is two-fold. First, they are considerably lighter weight than the giant column-type spokes used before. Second, they are also considerably more comfortable because they do flex some under loading, how much can be tuned by the number, material, lacing pattern, and tension of the spokes. So there's a much greater control over the characteristics of the wheel with modern spokes than the giant column-type spokes of yesteryear.

In addition to carrying the load, bicycle and motorcycle wheels have to handle the transfer of power. On a wagon or chariot, the wheels just respond and have to roll. On a bicycle or motorcycle, the power from the rider or engine is transmitted to the hub, forcing the hub to rotate. The spokes then need to transfer that power to the rim to make the wheel spin. This shearing rotation is why rear wheels rarely have a radial spoke pattern and instead have spokes that are at various angles to the hub.

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A spoke doesn't even need to be rigid, see: fiberfixspoke.com –  Martin Beckett Jan 16 at 22:40
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@ArjunNagarajan More isn't always "better." Traditional bicycle wheels are built with 32 spokes, but racing wheels can get as low as 10-12 and are often just as stiff due to material selection. Plus more adds weight that has to rotate, so less power goes to moving forward and more goes to spinning the wheel. So there are tradeoffs to make. –  tpg2114 Jan 16 at 23:29
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@RBerteig Also true. Specialized (who later sold it to Hed) make a tri-spoke wheel that is back to the column-type wheel rather than spoked wheels and disc wheels are commonly used that have no spokes but are solid instead. But covering that in the answer is tangential and really off-topic here, no matter how many of them I may own and race on the velodome :) –  tpg2114 Jan 17 at 2:25
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+1 I can't say I've ever sat down and thought about exactly how spokes work, and the exact forces applied to them. The weight-bearing attribute seems obvious, but I certainly never considered that the angled spokes are due to the shear force due do pedaling. –  Jonathon Reinhart Jan 17 at 7:36
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It was after decades of cycling that I realised that the weight of the bicycle is actually partly suspended from the top rim. The weight calculations are easy - forces on the axle and where the rubber meets the road must be equal. And as long as all spokes are under tension, torque also gets distributed evenly. –  Henk Langeveld Jan 17 at 11:49

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